1. Field of the Invention
The present invention relates to a liquid crystal display device, in particular, relates to a liquid crystal display device having liquid crystal and a switching element, the liquid crystal having an inherent spontaneous polarization or a spontaneous polarization induced by application of an electric field.
2. Description of Related Art
The liquid crystal display devices (LCD) are characterized in low power consumption, lightweight or the like. Because of these characteristics, they have been used for display devices of such as note type personal computers, portable display terminals or the like. So far, liquid crystal display devices of TFT-TN scheme that have thin film transistors (TFTs) which use amorphous silicon (a-Si) as switching element, and employ nematic liquid crystal have been used for displays of information terminals of approximately 15 inches.
However, such liquid crystal display devices have narrow viewing angle and slow response speed. Accordingly, they are insufficient for application to large size displays such as monitors or the like.
Recently, as a display method capable of solving these problems of the liquid crystal display devices, a display method that employs liquid crystal having an inherent spontaneous polarization or a spontaneous polarization that is induced by application of an electric field is gaining attention. As such liquid crystals, ferroelectric liquid crystal (FLC), anti-ferroelectric liquid crystal (AFLC), distorted helical ferroelectric liquid crystal (DHFLC), twisted ferroelectric liquid crystal (TFLC) or the like can be cited.
A display method employing surface stabilized ferroelectric liquid crystal (SSFLC), one method using those liquid crystal materials, unwinds a helical structure of chiral smectic C phase of smectic liquid crystal through an interaction between an alignment layer and liquid crystal. Due to the torque generated by the interaction between a spontaneous polarization induced at that time and an electric field, switching is carried out. To display a gray scale by use of these liquid crystals, active elements such as TFTs are necessary to be employed.
On the other hand, a display method that employs a liquid crystal phase (SmCa phase) of an anti-ferroelectric liquid crystal is known. In this method, other than two stable states of the ferroelectric liquid crystal, during zero applied voltage, an anti-ferroelectric liquid crystal structure is taken. Recently, according to this method, there appeared a report disclosing that, without using switching elements, the gray scale can be reproduced (N. Koshoubu, K. Mori, K. Nakamura, and Y. Yamada: Ferroelectrics, 149 (1993), p. 295).
To this method, a method employing chiral smectic C liquid crystal together with switching elements consisting of an active element has been recently proposed (J. Funfschilling and M. Schadt: J. Appl. Phys. 66 (1989), p. 3877).
A display device employing this method has advantages such as shown in the following various points over the aforementioned methods.
(1) Excellent in display capability of gray scale. That is, in this method, the transmittance varies relatively smoothly with respect to an applied voltage. Further, contrary to a liquid crystal display device employing a surface stabilized ferroelectric liquid crystal, there is no difficulty in displaying the gray scale.
(2) The liquid crystal devices of this display method are capable of being driven at a low voltage (0 to 5 V). Accordingly, the liquid crystal display device of low power consumption can be materialized.
(3) The liquid crystal display device of this display method is resistant to a mechanical shock. Accordingly, it is not likely to induce alignment destruction due to the mechanical shock like the surface stabilized ferroelectric liquid crystal.
Here, for an anti-ferroelectric liquid crystal that is one example of a liquid crystal material that has an inherent spontaneous polarization or spontaneous polarization induced by application of an electric field (hereinafter referred to as liquid crystal having a spontaneous polarization), the relationship between an alignment and an electric field is shown in FIG. 21.
Molecules 51 of this anti-ferroelectric liquid crystal in a state A during zero applied voltage align in staggering manner to cancel out the spontaneous polarizations each other. At this time, an average optical axis 52 of liquid crystal molecules 51 align in a longitudinal direction. Therefore, when two polarizers are disposed in the same direction and an orthogonal direction with respect to the optical axis 52 to be in a crossed Nicols state as shown with arrows 53 and 54 in the figure, it becomes a dark state (normally black) in A state. However, in B state or C state realized when a positive or a negative voltage is applied, following the direction of an electric field 55, the molecules 51 of the anti-ferroelectric liquid crystal are aligned in one direction to deviate the optical axis 52 from the polarizing direction of the polarizers, resulting in a highlight state. According to the positive and negative voltages applied to the molecules 51 of the anti-ferroelectric liquid crystal, as B state and C state, the directions of the spontaneous polarization are different each other. Accordingly, during inversion of the polarity, an electric charge is needed to change the direction of the spontaneous polarization. That is, the anti-ferroelectric liquid crystal differs in alignment states of the liquid crystal between under the positive voltage and the negative voltage, and requires an electric charge to change the direction of the spontaneous polarization during the inversion of the polarity. This point differs from the nematic liquid crystal.
Further, in threshold-less anti-ferroelectric liquid crystal (TLAFLC), according to the intensity of the voltage applied between the electrodes, other than three alignment states of a zero applied voltage state (A state), a positive voltage applied state (B state) and a negative voltage applied state (C state), any arbitrary aligning state between these states is possible. Therefore, by applying in a display device of an active matrix scheme in which switching elements such as TFTs are formed in pixels disposed in matrix, and by constituting to maintain the voltage that gives the aforementioned arbitrary alignment state during the non selection time period, gray scale display is made possible.
First, in a display element in which, threshold-less anti-ferroelectric liquid crystal material is sandwiched by electrodes, and, polarizers are disposed crossed-Nicols, the relationship between an electric voltage V applied between electrodes and light transmittance intensity T in an equilibrium is shown in FIG. 22. Incidentally, the applied voltage is a sum of the voltage applied to the liquid crystal material and the voltage applied to an alignment film disposed on the electrodes to align the liquid crystal.
Since the alignment of the liquid crystal varies due to the applied voltage V, as shown in this figure, the intensity of the light transmittance T becomes large approximately in proportion to V. A saturation voltage is set as V.sub.sat. When V takes +V.sub.sat and -V.sub.sat, the alignment of the liquid crystal becomes B or C state as shown in FIG. 21, respectively, resulting in the maximum light transmittance T.sub.max. Hereinafter, the maximum value T.sub.max of the intensity of the transmitted light is referred to as the maximum brightness.
The DHFLC is known that by expressing with an equivalent circuit, the electrical and electrooptical response can be described with accuracy. The equivalent circuit of the DHFLC is shown in FIG. 23. Here, C.sub.LC 56 represents the dielectric response portion of a fast component of the capacitance of the liquid crystal, C.sub.hX 57 represents the dielectric response portion of a slow component of the capacitance of the liquid crystal, C.sub.series 58 represents a capacitance of the alignment film portion, and R.sub.series 59 represents a resistance of an electrode by which a voltage is applied to the liquid crystal material, respectively. As to the slow component, for expressing its delay, a resistance component expressed by R.sub.hX 60 is disposed in series with C.sub.hX 57. The C.sub.hX 57 component is construed as a capacitance component corresponding to the spontaneous polarization P.sub.s. The electric charge Q.sub.hX accumulated at the both ends of C.sub.hX 57 can be construed to be approximately proportional to the angle of rotation of the liquid crystal molecule. In the case of the polarizers being set crossed-Nicols, when the electric charge accumulated at the both ends of C.sub.hX 57 is zero, it is a black level. As the electric charge increases, the brightness rises. It is construed that, when the electric charges corresponding to P.sub.s is accumulated, the maximum brightness is reached.
By carrying out a circuit simulation by use of such an equivalent circuit, the electrical and electrooptical response can be obtained accurately. For the circuit simulation, a SPICE (Simulation Program with IC Emphasis, U. C. Berkley) and so on, for instance, can be employed. Further, even when such an equivalent circuit model is applied for a threshold-less anti-ferroelectric liquid crystal, its electrical and electrooptical response can be described accurately.
As a method for driving, by an active-matrix scheme, a liquid crystal display device in which liquid crystal having a spontaneous polarization is sandwiched between a pixel electrode disposed in matrix and a counter electrode, there are a frame inversion driving method, a reset driving method and so on. Further, as a method for driving, by the active-matrix scheme, a liquid crystal display device in which a nematic liquid crystal is sandwiched between a pixel electrode and a counter electrode disposed in matrix, there are a frame inversion driving method and so on.
A constitution of one pixel for the case of the driving by the frame inversion scheme is shown in FIG. 24. The driving waveforms are shown in FIG. 25.
In this array, gate lines 61 and signal lines 62 are disposed orthogonally, and in the neighborhood of the intersection thereof, switching elements (TFTs) 63 are disposed to write in signals. The source of each switching element 63 is connected to a pixel electrode 64, and a drain is connected to the signal line 62, respectively.
A frame period T.sub.frame during which the switching element is selected by applying the gate signal is normally 1/60 second. For every period, during the gate selection time T.sub.gon, the gate line is selected on state. When the number of the gate lines is Ng, the gate selection time T.sub.gon corresponds to the time obtained by dividing T.sub.frame by Ng. On the other hand, to the signal line, the voltage of which polarity reverses with the period identical as the frame frequency is applied. The voltage of the signal line has a central value of V.sub.sig-c. When being positive polarity, V.sub.sig-p is applied, and when being negative polarity, V.sub.sig-n is applied. The potential of the counter electrode is a constant of V.sub.com. During the gate selection time T.sub.gon, the TFT becomes on-state, the signal is written in the pixel electrode, and thereby the alignment of the liquid crystal material is controlled. That is to say, the liquid crystal changes the direction of the spontaneous polarization for each frame period. For the signal line inversion drive and the dot inversion drive, similar waveforms are obtained. The polarizers need only be disposed to be normally black.
On the other hand, in a reset driving, to the pixel electrode, the reset signal is written in immediately before the addressing of the signal so that the alignment of the liquid crystal becomes the black level state. By resetting thus the alignment of the liquid crystal, the afterimage can be prevented from occurring. That afterimage is induced by variation of the alignment after the addressing due to the influence of the alignment preceding the addressing. Methods for carrying out the reset driving will be described in the following.
Driving waveforms of the first method of the reset driving are shown in FIG. 26. In FIG. 26, a reference mark (a) shows the potential of the gate line and a reference mark (b) shows the potential of the signal line. As an array structure, the constitution shown in the aforementioned FIG. 24 is adopted.
For every frame period T.sub.frame, the gate line is selected for an on state during T.sub.gon0. When the number of the gate lines is Ng, the T.sub.gon0 corresponds to the time obtained by dividing T.sub.frame by Ng. The T.sub.gon0 during which the gate electrode is on-state is consisting of two parts, the former being the reset time T.sub.r, and the latter being the gate selection time during which the display signal is written in the pixel electrode, that is, the signal addressing time T.sub.gon.
The voltage applied to the signal line, during the reset time T.sub.r, is the potential V.sub.com of the counter electrode, and during the signal write-in time T.sub.gon, is the signal voltage that is to be written in the pixel. During the T.sub.gon0, the TFT is on state. During the former half of T.sub.gon0, that is, during the reset time T.sub.r, the pixel potential becomes approximately equal as the potential of the counter electrode V.sub.com. During the latter half of T.sub.gon0, that is, the signal write time T.sub.gon, the signal is written in the pixel electrode, to control the alignment the liquid crystal. The liquid crystal changes the direction of the spontaneous polarization for every frame period, thereby, the image sticking can be prevented from occurring. The polarizers need only be disposed to be normally black.
An equivalent circuit for one pixel of the second method of the rest driving is shown in FIG. 27. As the array constitution, in addition to the constitution of the frame inversion driving, there are a switching element 65 for resetting consisting of a TFT, and a reset line 66 for controlling on/off switching of the TFT 65. For the TFT 65 for resetting, a source is connected to a storage capacitance line 67, and a drain is connected to a pixel electrode 64.
The driving waveforms of the circuit are shown in FIG. 28. In FIG. 28, reference marks (a), (b) and (c) denote potentials of a gate line, a reset line and a signal line, respectively.
For every frame period T.sub.frame, during the gate selection time T.sub.gon, the gate line is selected on state. When the number of the gate line is Ng, the gate selection time T.sub.gon corresponds to the time obtained by dividing T.sub.frame by Ng. Immediate before the selection of the gate line, the reset line is selected on state during the reset time T.sub.r. Since the potential of the storage capacitance line is maintained at the potential approximately equal to V.sub.com of the counter electrode, during the reset time T.sub.r, the pixel potential becomes approximately equal to that V.sub.com of the counter electrode. During the gate selection time T.sub.gon thereafter, the signal is written in the pixel electrode and the alignment of the liquid crystal material is controlled. The liquid crystal changes the direction of the spontaneous polarization for every frame period, thereby, the image sticking is prevented from occurring. The polarizers need only be disposed to be normally black.
A liquid crystal display device having a nematic liquid crystal or liquid crystal having a spontaneous polarization that is interposed between a pixel electrode that are disposed in matrix and a counter electrode is driven by an active matrix scheme with a frame inversion method. The voltage applied to one arbitrary pixel and light transmittance thereof in that time are shown in FIG. 29. Incidentally, the polarizers are disposed to be normally black.
First, in a liquid crystal display device that has a nematic liquid crystal interposed, as shown in FIG. 29(a), gate signals are periodically applied from a gate line. At this time, the frequency of the gate signals is a frame frequency Ff and is normally 60 Hz. That is, the period of the gate signal T.sub.frame is normally 1/60 second (=16.67 ms). Whereas, to the signal line, as shown in FIG. 29(b), a voltage of which polarity reverses periodically with the same period with the frame period T.sub.frame is applied. Here, the voltage of the signal line has the central value of V.sub.sig-c, for the positive polarity, V.sub.sig-p is applied, and for the negative polarity, V.sub.sig-n is applied. In addition, to prevent flicker and image sticking from occurring, the potential of the counter electrode is kept at V.sub.com that is approximately 1 V lower than V.sub.sig-c.
Thus, when the voltage is applied on the gate during the gate selection time T.sub.gon that is the time during which the gate line is selected on-state for addressing the display signal to the pixel electrode, during the T.sub.gon, the switching element becomes on-state. The voltage of the aforementioned signal line is supplied to a pixel electrode through a switching element as a write voltage as shown in (c) of the same figure. Then, due to the write voltage supplied to the pixel electrode like this, a liquid crystal cell and a storage capacitance function as condensers. Accordingly, as shown in (d) of the same figure, a holding voltage of the nematic liquid crystal cell hardly shows lowering of the holding rate and is kept at an approximately constant value. That is to say, when an impurity is mixed in the liquid crystal, the holding voltage shows a decrease. However, when a fluorine-based liquid crystal that hardly contains an ionic impurity is employed, the holding voltage is maintained at an approximate constant. The light transmittance of the liquid crystal cell in this case is shown in (e) of the same figure. Since the nematic liquid crystal is low in response speed, the light transmittance builds up slowly. However, whichever positive polarity or negative polarity does the voltage of the pixel electrode have, the alignment of the liquid crystal is not affected, accordingly, the later light transmittance becomes approximately constant.
On the other hand, in the liquid crystal having a spontaneous polarization, when, due to input of the gate signal from the gate line shown in FIG. 29(a) and a voltage applied to the signal line shown fin FIG. 29(b), a write voltage shown in FIG. 29(c) is supplied to the pixel through the switching element, as shown in FIG. 29(f), the holding voltage of the liquid crystal cell decreases after the gate selection time, resulting in an extremely poor holding characteristic. In this case, the light transmittance of the liquid crystal cell becomes as shown in FIG. 29(g).
Thus, when liquid crystal having a spontaneous polarization is active matrix driven (holding drive), the light transmittance does not increase like that in a static drive. As a result of this, the liquid crystal display device that employs liquid crystal having the spontaneous polarization shows a lowering of the contrast and a deterioration of the display quality.
As a result of the detailed study of the problems by the inventors, it is found that the lowering of the contrast is caused by the following reasons.
That is, in the case of active matrix drive, as shown in FIG. 29(c), the voltage supply to write in one frame is executed during only a part of one frame. Normally, in dielectric response of liquid crystal having the spontaneous polarization, there are a fast component and a slow component. The fast component has a saturation time constant of several .mu.s or less. On the contrary, the saturation time constants of the slow components are generally 80 .mu.s is or more in many cases. Further, the saturation time constant of the storage capacitance is also several .mu.s or less. The typical gate selection time T.sub.gon is 64 .mu.s or less. Accordingly, the response of the fast component and the storage capacitance completes within the gate selection time. However, the response of the slow component does not complete.
The change of alignment of a liquid crystal molecule corresponds to the response of the slow component of the dielectric response. Accordingly, the change of the alignment of the liquid crystal molecule does not complete within the gate selection time T.sub.gon. Therefore, during the rest of frame period after the gate selection time, charges accumulated at the dielectric response part of the fast component of the liquid crystal capacitance and at the storage capacitance move to the dielectric response part of the slow component, thus the change of the alignment of the liquid crystal molecules continues to occur. Accordingly, as shown in FIG. 29(d), the holding voltage decreases. At this time, in the case of the holding voltage being the voltage lower than the saturation voltage V.sub.sat, the transmittance decreases compared with the static driving, accordingly the brightness during white level becomes lower than the maximum brightness T.sub.max, resulting in the lower contrast.
Incidentally, the response time of liquid crystal normally indicates a time during which, when a voltage is applied to liquid crystal in a certain alignment state to change the alignment state, the amount of the change of the transmittance of the liquid crystal cell from the time of application reaches 90% of the difference of the transmittances of before and after application. Here also adopts this definition. In the liquid crystal having the aforementioned spontaneous polarization, since the optical response is due to the dielectric response of the slow component corresponding to the change of the alignment of the liquid crystal, the response time is in many cases 80 .mu.or more.
In the nematic liquid crystal that does not possess spontaneous polarization, a liquid crystal molecule responds with respect to the absolute value of an applied voltage. That is, +5 V and -5 V applications induce the same alignment. Therefore, even if the first frame upon from off to on switching gives rise to an insufficient alignment change, as goes on to second frame, third frame, and so on, gradually the alignment of the liquid crystal molecules continues to change. Thus, after several to several tens frames, the alignment becomes identical as the case of the same voltage being applied by the static driving scheme. That is, after several to several tens frames, there is the same transmittance with the static-driving scheme.
On the other hand, the liquid crystal possessing a spontaneous polarization differs in alignment of the liquid crystal molecule according to the polarity of the applied voltage. That is, +5 V and -5 V applications give rise to different alignments. Therefore, when driven by a frame inversion scheme, at the first frame upon from off to on switching, the liquid crystal molecule takes on an alignment of either polarity (for instance, positive polarity). However, being slow in the response speed, the alignment of the case where the same voltage is applied by the static drive scheme can not be attained. In the second frame, upon inversion of the polarity, the liquid crystal molecule changes its alignment from the alignment where the first frame is positive polarity through the alignment under zero voltage input. Accordingly, similarly as the first frame that is switched from off state to on-state, the alignment of the static-driving scheme can not be attained. Further, since the polarity is reversed for every frame, even in the later frame, the alignment of the static-driving scheme under the same voltage application can not be attained. As a result of this, the transmittance, compared with the static-driving scheme, largely deteriorates, resulting in low contrast display.
To the liquid crystal having a spontaneous polarization, the reset driving is carried out. For this, the voltage applied to the arbitrary pixel and the light transmittance are shown in FIG. 30.
When the voltages are applied to the gate line and the signal line with the driving waveforms shown in FIG. 30(a) and (b), respectively, the pixel potential varies as shown in FIG. 30(c), and the light transmittance of the liquid crystal cell varies as shown in FIG. 30(d).
When driven by the reset drive, since the state is reset in the black level before the addressing of the signal, the alignment of the liquid crystal molecule varies small during the signal addressing compared with the case of the frame inversion drive. However, as shown in FIG. 30(d), as identical as the frame inversion drive, after the gate selection time, the electric charges move and the holding voltage becomes low, and when this voltage becomes lower than V.sub.sat, the contrast becomes low.
An equivalent circuit is shown in FIG. 31 wherein a storage capacitance C.sub.s 68 and a TFT 69 are added to the equivalent circuit shown in FIG. 23. This drawing can be construed as an equivalent circuit showing an electrical and electrooptical response of one pixel. As mentioned above, C.sub.LC 56 denotes the dielectric response of the fast component of the capacitance of the liquid crystal, and C.sub.hX 57 denotes the dielectric response of the slow component of the capacitance of the liquid crystal, respectively.
The source 70 of the TFT 69 is connected to the pixel electrode, and the drain 71 is connected to the signal line. Between the gate 72 and the source 70, a parasitic capacitance C.sub.gs 73 exists. In this circuit, corresponding to the reset drive, when an operation is considered where the display preceding the addressing is black level and the display after the addressing is white level, during the gate selection time, the potential of the pixel electrode V.sub.pix approaches the potential of the signal line V.sub.sig. When the addressing capability (that is expressed by a conductance of the TFT 69) during the gate selection time is large, upon completion of the gate selection time, V.sub.pix is approximately equal to V.sub.sig. However, when the addressing capability is small, the differences of V.sub.pix and V.sub.sig upon completion of the gate selection time do not become sufficiently small.
Further, even when the addressing capability is large, during the gate selection time, the responses due to C.sub.s 68 and C.sub.LC 56 nearly complete but the response due to C.sub.hX 57 does not complete. After the gate selection time, the electric charge stored at C.sub.s 68 and C.sub.LC 56 moves to C.sub.hX 57 to approach an equilibrium. When the electric charge stored at the C.sub.hX 57 at the equilibrium is smaller than that equivalent to the spontaneous polarization PS of the liquid crystal, the brightness becomes lower than the maximum brightness T.sub.max.
From the above, in a liquid crystal display device involving liquid crystal having a spontaneous polarization, the problem that the brightness becomes low during the white display is considered to be induced by combination of the following two factors. One is a part due to the physical properties of the material such as the spontaneous polarization P.sub.s of the liquid crystal and a response time .tau., and the other is a part due to the pixel parameter such as the addressing capability of the TFT and the magnitude of the storage capacitance C.sub.s.
With an object to obtain a liquid crystal display element of high contrast, in order for the absolute value of the voltage applied between the electrodes sandwiching the liquid crystal layer to be the saturation voltage V.sub.sat or more at the equilibrium, the following methods can be thought of.
(I) The gate selection time T.sub.gon for addressing the display signal to the pixel electrode is set longer than the response time .tau. of the liquid crystal cell. PA0 (II) The storage capacitance C.sub.s and the capacitance of the fast component of the dielectric response of the liquid crystal are made large. PA0 (III) The maximum value of the applied voltage between the electrodes sandwiching the liquid crystal layer is made large.
Among them, as to the method of (I), the magnitude of the gate selection time T.sub.gon is restricted by the definition of the display element. Accordingly, for the display element of high definition, T.sub.gon can not be made large.
Here, the response time .tau. of a liquid crystal cell is defined, as mentioned above, in the following way. That is, the response time is the time during which after the application of the voltage, the amount of change of the transmittance reaches 90% of the difference between the transmittances of the liquid crystal cell before and after the voltage application. On the other hand, in an electric circuit, the response speed is usually defined as the time during which the voltage becomes 1-exp (-1) times the applied voltage. Accordingly, when that time is set as .tau.e (s), the transmittance can be considered to vary due to the response of the potential, and .tau. and .tau.e have a relation expressed by the following equation. EQU .tau.=.tau.e.times.log.sub.e 10.
In liquid crystal having a spontaneous polarization, the response time .tau. of a liquid crystal cell is determined by the physical properties of the liquid crystal material, the capacitance of an alignment layer or the like. At the present time, the condition of (I) that requires to make the response time of a liquid crystal cell smaller than the gate selection time T.sub.gon is not yet achieved.
Further, as to the method of (II), the magnitude of the storage capacitance C.sub.s, being restricted by the size of one pixel and the array structure, can not be made large without limit. In addition, the capacitance of the fast component of the dielectric response of liquid crystal, being determined by the physical properties of the material, can not be made large exceeding a certain limit. Further, to make large the storage capacitance C.sub.s and the capacitance of the fast component of the dielectric response of the liquid crystal corresponds to that the capacitance to be written by a switching element that is a TFT is large. However, when an on-resistance while a TFT is selected is large and the addressing capability is small, the amount of the current is restricted by the resistance. Accordingly, there occurs a problem that, while the TFT is selected, the electric charge is not fed sufficiently and the potential of the pixel electrode does not sufficiently respond. Therefore, the contrast improvement due to the method of (II) is also difficult.
Further, the voltage of the applied signal is restricted by the voltage resistance of a driver, therefore, there is an upper limit. Accordingly, an improvement of the contrast due to the method of (III) is also difficult.
In addition, since the liquid crystal molecule responds to a torque induced through an interaction between a spontaneous polarization and an electric field, when a response time of liquid crystal having the spontaneous polarization is put as .tau..sub.LC, the following relation generally holds between the .tau..sub.LC and the spontaneous polarization P.sub.s, EQU .tau..sub.LC =.eta./P.sub.s
where, the .eta. is a constant determined by viscosity of the liquid crystal and so on. That is, when the spontaneous polarization P.sub.s is too small, the response time .tau..sub.LC of the liquid crystal becomes large, resulting in a problem that the display of excellent response can not be obtained.
Further, a display device employing liquid crystal having a spontaneous polarization, by making longer a period T.sub.s of polarity inversion of the voltage applied between the pixel electrode and the counter electrode (common electrode) than the frame time T.sub.frame, can be driven to satisfy the following relation between the response time of the liquid crystal .tau..sub.LC and the gate selection time T.sub.gon, EQU T.sub.s .gtoreq.T.sub.frame .times.(.tau..sub.LC /T.sub.gon),
thereby a high contrast display can be obtained. This method that is called a quasi DC drive scheme is proposed (Japanese Patent Application No. 8-235571).
However, in this method, the response of the liquid crystal is not assumed to complete within the time of one frame. Accordingly, there is a problem that the response time is slow.
In addition, the liquid crystal that possesses the spontaneous polarization requires much electric charge to align. Thus, since a capacitance where a TFT that is a switching element has to write in becomes large, in the case of the gate of the TFT being selected, if the electric current flowing to the channel layer is small, enough amount of the electric charge can not be supplied during the selection period. Therefore, there occurs a problem that a write deficient voltage that is the difference between the pixel and the signal line potentials upon completion of the gate selection time T.sub.gon becomes large.
In a liquid crystal display device employing a nematic liquid crystal, the write deficient voltage is designed to be 100 to 200 mV at maximum, therefore a display of excellent quality is obtained. However, if the write deficient voltage is large, an angle of rotation of the liquid crystal molecule during the gate selection time T.sub.gon becomes small to result in the lowering of the transmittance compared with the static drive. In addition to the lowering of the contrast, the nonuniformity of the amount of the write deficient voltage due to the gate wiring delay and so on in a panel becomes large to result in a problem that the homogeneity of the contrast in a panel deteriorates.
On the other hand, a TFT that is a switching element is constituted of thin insulating films, metallic films and semiconductor films. When a high voltage is applied between the electrodes, due to electrostatic destruction, the element becomes nonfunctional to induce display error. Accordingly, the amount of the current that flows the channel layer of a switching element is required to be suppressed down to a certain value or less. Further, when a storage capacitance C.sub.s is large and a capacitance thereto a TFT writes in is large, there is a problem that the amount of electric charge necessary for charging and discharging increases to result in an increase of power consumption.
Further, in a liquid crystal display device having liquid crystal having a spontaneous polarization, upon changing the voltage applied to the wiring of the gate line and signal line, a large quantity of electric charge moves during the shift of the signal. Accordingly, compared with the device involving the nematic liquid crystal, the applied signal pulse tends to be distorted. The distortion of the signal pulse induces nonuniformity of the electric charge to be written in a plane. Thus, the potential of the common electrode for suppressing the image sticking (due to the direct current component of the voltage applied to the liquid crystal layer) and flickering becomes different in the plane. Therefore, there is a problem that the display uniformity in the plane deteriorates largely.
Still further, the inventors have studied, in a liquid crystal element employing liquid crystal having a spontaneous polarization, a relation between a signal voltage and a pixel potential in an active matrix drive scheme, and obtained the following result.
That is, when a signal voltage V.sub.sig is applied as shown in FIG. 32(a), in a holding period (T.sub.H) during the off-time of the gate potential shown in FIG. 32(b), the pixel potential V.sub.pix becomes lower by a feed-through voltage .DELTA.V.sub.p as shown in FIG. 32(c). This is considered a phenomenon occurring due to the existence of the parasitic capacitance C.sub.gs between the gate and the pixel electrodes in a TFT.
First, when the gate is on-state, the gate potential becomes V.sub.gon and to the pixel electrode V.sub.sig is written in, and, in parasitic capacitance C.sub.gs between gate and pixel electrodes, storage capacitance C.sub.s, and capacitance C.sub.cell consisting of liquid crystal and an alignment film, the electric charges corresponding to the respective potentials are stored. Thereafter, the gate becomes off state. When the gate potential is V.sub.goff and the pixel potential is floating, the stored electric charge is redistributed to C.sub.gs, C.sub.s, C.sub.cell under this condition. Thereby, upon the gate becoming off-state, the pixel potential decreases, that is, a feed-through phenomenon occurs.
This phenomenon is also observed in a liquid crystal display device of a TFT-TN mode employing nematic liquid crystal. However, because of the slow response speed of the liquid crystal, the optical response varies smoothly during the holding period, resulting in less discernibility of the time averaged brightness difference between the polarities. On the other hand, in a liquid crystal display device involving liquid crystal having a spontaneous polarization, the liquid crystal instantaneously responds to the voltage put on the liquid crystal and the alignment film. Therefore, according to the polarity of the applied signal voltage, the brightness level differs remarkably. Thus, the difference in the display brightness levels according to the polarity of the signal voltage, though depending on the period of the polarity inversion, causes a problem that the display defect such as flickering and so on occurs. In addition, since the voltage put actually on the liquid crystal and alignment film differs according to the polarity, the deterioration of the display quality such as the so-called image sticking and so on are likely to occur.
The present inventors have studied a relationship between such a feed-through voltage and a signal voltage in detail and found the following result.
FIG. 33 is a diagram showing the signal voltage dependence of the feed-through voltage .DELTA.V.sub.P in a liquid crystal display device employing liquid crystal having a spontaneous polarization.
As obvious from this figure, the value of the feed-through voltage V.sub.P is always positive, and is symmetrical with respect to the polarity of the signal voltage. Further, as the absolute value of the signal voltage becomes large, the feed-through voltage increases.
This result shows an opposite tendency with respect to the relation of the liquid crystal display device that employs the conventional nematic liquid crystal. Since, as the signal voltage increases, the difference of the pixel potential between the polarities becomes large, the deterioration such as the image sticking or the like is considered to be liable to proceed. Further, it is found that the signal voltage dependence of the feed-through voltage is remarkably large compared with a device that employs nematic liquid crystal, thereby, the difference of the brightness level becomes large between the polarities.